The expression goes back to the battle of Waterloo, fought on June 18, 1815 in Belgium between the British army helped by the Prussian army against the French army led by Napoléon the First. The French army was defeated, but neither Napoléon nor anyone who fought said “Waterloo, morne plaine.”

Like my cousin with Waterloo I’ve never heard the expression using Vietnam:)
Thank you for teaching me a new one!
And the way I say Waterloo is just what I heard living in France. These things don’t go away:)